1. Field of the Invention
A CMOS transistor and an AlGaN/GaN amplifier are formed on the same substrate, where the substrate has high thermal efficiency.
2. Description of the Related Art
Gallium nitride (GaN) is a wide-bandgap semiconductor material that has potential applications in high-speed, high power transistor devices. One of the main drawbacks to the production of these devices is the limited availability of suitable substrates for epitaxial growth. A high-quality bulk single crystal substrate at low cost that has a large area is desirable for the growth of gallium nitride epitaxial layers for device fabrication. In one example of the related art technology, the GaN epitaxial layer would be grown homoepitaxially on a single crystal GaN substrate. However, the cost and availability of these wafers are prohibitive.
Currently, GaN films are produced by heteroepitaxial growth on either single crystal silicon carbide (SiC) or sapphire. Due to the lattice mismatch between GaN (4.8 Å) and sapphire (4.763 Å) or 4H-silicon carbide (3.0730 Å), a significant number of threading dislocations on the order of 108 are formed during the growth process.
Another substrate of interest is single crystal silicon, which is readily available in sizes up to 12 inches in diameter. However, silicon does not have the thermal dissipation properties that are necessary for high power, high-speed devices. In addition, Si and GaN have a significant thermal expansion mismatch. One potential method to reduce the cost and improve the properties of the substrates is to manufacture the devices on a polycrystalline substrate utilizing three-dimensional integration through wafer bonding.
Wafer bonding allows heterogeneous substrates to be bonded together at temperatures as low as 200° C. Low temperature bonding is important to minimize chemical reactions of the metals and stresses that arise due to thermal coefficient of expansion mismatches. Wafer bonding occurs when wafers with atomically smooth surfaces are brought into contact and initially adhere due to hydrogen bonding, which is a result of the reaction between water molecules and hydroxyl groups present on the wafer surfaces. Subsequent anneals either transport the water away from the interface or cause the water to react and produce a siloxane bond across the interface.
The siloxane bond, Si—O—Si, is a covalent bond. In the case of silicon-to-silicon bonding, where no siloxane bond is desired, high temperature anneals will cause the oxygen to diffuse away from the interface, resulting in Si—Si covalent bonding.
A conventional approach to forming a multi-layered substrate is typified by the work F. J. Kub et al. (U.S. Pat. Nos. 6,328,796 and 6,497,763). This related art technology forms a composite substrate that includes polycrystalline layers, amorphous layers and single crystal layers. However, the conventional technology requires an oxide bonding layer in order to have monocrystalline silicon bond to polycrystalline substrate structure. Alternately, the conventional art used carbonization (which can produce impurities) to promote adhesion.
Also, related art technology necessitates that when forming devices that combine high speed silicon switching circuits with high power AlGaN/GaN amplifier circuits, these two different types of circuits be formed on different chips. Then the chips are formed into a circuit using such techniques as wire bonding. However, this approach is inefficient and results in yield loss arising from the extra manufacturing steps required to manufacture two chips and wire them together.
Accordingly, the development of high power semiconductor devices requires new and low cost substrates having both good thermal conductivity and superior electrical properties. Additionally, new engineered substrates are needed that can accommodate circuit technologies based on different material types such as Si and ALGaN/GaN.